People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Why Red Hair And Blue Eyes Is So Rare, Plus 4 Other Surprising Facts About Redheads

We can more easily spot a redhead in a crowd than a blonde or brunette. They are few and far between; about one to two percent of the population possesses natural red locks. However, their hair isn't the only thing that sets them apart - genetic differences can affect everything from their pain threshold to how easily they bruise.

In the Reactions video, "The Chemistry of Redheads," the American Chemical Society explains redheads get their hair color and pale skin due to a genetic variant that causes their cells to produce reddish pheomelanin — a type of melanin. Redheads have a particularly high abundance of pheomelanin, with very little eumelanin, which ranges from brown to black. On the surface of melanocytes is a protein known as melanocortin 1 receptor, or MC1R. When it's activated, it causes melanocytes to specifically produce eumelanin over phenomelanin, which balances out an individual's ratio.

Contrastingly, redheads are born with a genetic variant that causes MC1R to chemically function differently on melanocytes, which leads to less eumelanin and more pheomelanin production. These genetic variants are recessive, meaning in order to be born a redhead, either non-redhead parents are carriers (25 percent), or one parent is a redhead and the other is a carrier (50 percent); or both parents are redheads (almost 100 percent). This makes redheads rare and unique both on the outside and inside.

Take a look at other facts about redheads that you probably didn't know.

Red Hair And Blue Eyes Is Rare

The combination of having both red hair and blue eyes is so rare because blue eye color is also a recessive trait, meaning both parents must carry the gene for a child to have it. Red hair occurs naturally in one to two percent of the human population, while just 17 percent of the world's population has blue eyes. The majority of redheads have brown, hazel, or green eyes.

Require More Anesthetic

Redheads may be harder to sedate than blondes or brunettes, and require more anesthesia during surgery. In a 2004 study, redhead patients required 20 to 30 percent more anesthetic than other women to reach the same level of sedation. The same experiment was also replicated with mice, which found animals with a MC1R mutation also required more sedation. Researchers suspect because the gene belongs to the family group of genes that play a role in pain, the mutation causes redheads to be more sensitive to it. The genetic mutation could affect the activity of endorphins, known as the body's natural painkillers, which may lead to an increase in pain.

Five fast facts about redheads, from bruising easily to being left-handed, that you probably didn't know.Photo courtesy of Pexels, Public Domain

Sensitive To Cold And Hot Sensations

Gingers are more likely to be sensitive to cold and hot sensations than people with different hair colors. Researchers at Louisville University in Kentucky found they needed extra doses of anesthetic during surgery, and were susceptible to pain at around 43 degrees Fahrenheit, unlike those with dark hair who didn't begin to flunch until the temperature got down to freezing. It's believed MC1R may cause the temperature-detecting gene to become over-activated, making redheads more cold.

More Likely To Bruise

Redheads are also more likely to bruise. They have normal blood counts and coagulate blood like others, yet a 2006 study found they bruise more easily. Researchers suspect people who always bruise excessively may assume this is normal, because they've experienced it often. However, people who bruise only rarely may become concerned about a single bruise linked with minor trauma. There are some normal, healthy people who consider their bleeding and bruising to be excessive.